1. Field of the Invention
The present invention relates to a charged particle beam writing method, a method for detecting the position of a reference mark for charged particle beam writing, and a charged particle beam writing apparatus.
2. Background Art
Semiconductor devices are being further shrunk in size. Along with such a trend, extreme ultraviolet (EUV) lithography has attracted attention in recent years. In the extreme ultraviolet lithography, extreme ultraviolet light is used as exposure light. Since the wavelength of the EUV light is as short as 13.5 nanometers, it is expected that a resolution in use will be enhanced. However, a mask material through which the EUV light can be transmitted without attenuation has not been found. Thus, not a transmissive mask but a reflective mask is typically used to perform the EUV lithography (refer to Japanese Patent Laid-open No. 2004-289110). The reflective mask has a multilayer film and an absorber pattern. The multilayer film is formed on the surface of a substrate and reflects light. The absorber pattern is formed on the multilayer film and absorbs light. In a process for forming the reflective mask, if an environmental contaminant is mixed into the multilayer film or the multilayer film is formed on an irregularity located on the surface of the substrate, then the ordered structure of the multilayer film may be disordered, resulting in the multilayer film having a phase defect. When light (such as the EUV light) having a short wavelength is used as exposure light, a phase defect may occur and affect exposure irrespective of an extremely small irregularity.
There is a known method for shifting the position of a portion, on which writing is to be performed, of the absorber pattern so that the phase defect is hidden under the absorber. In the method, a defect detection apparatus detects the position of the phase defect located in the multilayer film before the absorber is formed. Specifically, the position of the phase defect is detected on the basis of the position of a reference mark formed in the multilayer film. After that, the absorber and a resist are formed on the multilayer film. To write a pattern on the resist, the position of the reference mark is detected and the position of the phase defect is determined on the basis of the detected position of the reference mark. Then, a pattern is written on the resist so that the phase defect is covered with the absorber. For that reason, the position of the reference mark needs to be detected before the pattern is written by an electron beam writing apparatus.
The multilayer film is formed by alternately laminating silicon (Si) films and molybdenum (Mo) films. The multilayer film includes 40 to 50 films. When a predetermined portion of the surface of the multilayer film is irradiated with laser light, the Si film(s) and the Mo film(s) react in the predetermined portion to form a molybdenum silicide (MoSi) film. In this case, since the volume of the formed MoSi film is reduced compared with the total volume of the Si film(s) and the Mo film(s), a step portion having a height of 50 to 60 nm is formed between the surface of the multilayer film and the surface of the MoSi film, where a concave reference mark is formed.
There is a known method for detecting the position of the thus formed reference mark by means of an electron beam writing apparatus. In the known method, an electron gun emits an electron beam to scan the reference mark, and the position of the reference mark is detected on the basis of the waveform of a signal of an electron reflected on the reference mark.
Since a semiconductor process such as a lithographic process or an etching process is not used in forming the reference mark, the reference mark has a large roughness (edge roughness) on its edge. In addition, the electron beam has a very small spot diameter. Thus, the signal waveform of the electron reflected when the electron beam is scanned over the reference mark is affected by the edge roughness. When the spot of the electron beam is slightly shifted from the reference mark, a profile of the signal waveform of the reflected electron is changed, and thereby the position of the reference mark may not be reproducibly detected.